1. Field of the Invention
The present invention relates to a method of magneto-optically recording and erasing information, which allows overwriting to be carried out. It also relates to a magneto-optical information storage medium which is applied to the method.
2. Description of the Related Art
A thin film of rare earth-transition metal amorphous alloy (a-RE-TM) such as TbFe, TbCo, TbFeCo, GdTbFe and GdTbFeCo has an easy magnetization axis in a direction perpendicular to its surface and its magnetic characteristic changes in response to temperature. Magneto-optical information recording technique uses this characteristic of a-RE-TM film. In the magneto-optical information recording technique, recording of information is performed by following way:
Laser beam is irradiated onto a recording layer made of a-RE-TM to raise temperature at this beam-irradiated portion and reduce magnetic coercive force at the portion to a value smaller than external magnetic field, and a small reversed-magnetic domain is formed at the portion by the external magnetic field: erasing of information from the recording layer is performed by extinguishing this reversed-magnetic domain. Reproducing of information is performed by utilizing a magneto-optical effect such as the polar Kerr effect. This magneto-optical recording is superior in non-contact accessibility, exchangeability of medium and has high recording density, which are merits common to the optical recording. However, it has a demerit of making it more difficult to overwrite information, as compared with the magnetic recording.
Various kinds of overwriting techniques according to the magneto-optical information recording have been proposed. The most attractive one of them is of the optical modulation and one beam type (which is called one beam method).
As described in preliminarily-opened Japanese Patent Application Sho 62-175948, for example, the one beam method comprises the steps of previously applying initial auxiliary magnetic field to a magneto-optical information storage medium, which is provided with a recording/reproducing layer and an auxiliary recording layer, to orient magnetization in the auxiliary recording layer to a predetermined direction, and then irradiating pulse-like-modulated beam onto the medium to form bits whose magnetization is directed in one direction when the strength of the beam is high in level and bits whose magnetization is directed in the opposite direction when the strength of the beam is low in level. According to this technique, however, the recording/ reproducing layer and the auxiliary recording layer are exchange-coupled with each other. Exchange coupling force, which is difficult to control, must be therefore controlled in such a way that magnetic coercive forces of the recording/reproducing and auxiliary recording layers are in a specific relation to the exchange coupling force. This makes it difficult to make the medium. Further, magnetization in the auxiliary recording layer must be oriented to a certain direction before the overwriting operation. This makes it necessary to use a large-sized initial-magnetizing magnet which can apply several KOe.
The optical modulation one beam method has attracted attention like this as the overwriting technique according to the magneto-optical information recording manner, but it is complicated and difficult to control. Therefore, a magneto-optical information recording method enabling to perform overwriting which is suitable for practical use has not been realized yet.